Eyespot also knew as the stigma, It is a photoreceptive organelle which is mainly found in the motile form of green algae and in unicellular photosynthetic organisms for example euglenids.
Eyespot help in cell’s phototaxis, it senses the intensity and direction of light source and responds to it. In response, it helps the organism in swim towards the light (positive phototaxis), or away from it (negative phototaxis).
Most importantly its main function is, it find out optimal light conditions for the photosynthesis of cells.
- The light microscope view of the eyespot appears as dark with orange-reddish spots or stigmata.
- The carotenoid pigments help them to achieve their color which is contained within bodies known as pigment granules.
- They contain photoreceptors which are mainly found in the plasma membrane overlying the pigmented bodies.
- The electron Microscopic view appears as a highly ordered lamellar structure and is made by membranous rods in a helical arrangement.
- In Euglena the eyespot apparatus contains the paraflagellar body which connects the flagellum with the eyespot.
- In Chlamydomonas, it is a part of chloroplast which is takes on the appearance of a membranous sandwich structure.
- In Chlamydomonas they are made of chloroplast membranes (outer, inner, and thylakoid membranes) and carotenoid-filled granules which is covered by the plasma membrane.
- Stacks of granules on eyespot functions as quarter-wave plate and reflects the incoming photons back to the overlying photoreceptors, and also protects the photoreceptors from light coming from other directions.
- During the cell division, it dismantles and reformed again within the daughter cells in an asymmetric fashion in relation to the cytoskeleton which is essential for proper phototaxis.
Different Eyespot Proteins
- Eyespot senses light by using photoreceptor proteins, which are mainly found in unicellular organisms.
- These proteins has two main groups such as flavoproteins and retinylidene proteins (rhodopsins).
- The flavoproteins carries flavin molecules as chromophores.
- The retinylidene proteins carry retinal.
- The Euglena photoreceptor protein is a flavoprotein.
- The phototaxis in Chlamydomonas is carried out by an archaeal-type rhodopsins.
- Eyespot also contains different structural, metabolic and signaling proteins.
- In Chlamydomonas, the eyespot proteome is made of 200 different proteins.
The Photoreception and signal transduction
- The photoreceptor in Euglena is known as a blue-light-activated adenylyl cyclase, because on excitation these receptor proteins produces cyclic adenosine monophosphate (cAMP) as a second messenger. Then the chemical signal transduction alters the flagellar beat patterns and as a result, the cell movement is occurring.
- In Chlamydomonas the archaeal-type rhodopsins carry all-trans retinylidene chromatophore which is photoisomerization to a 13-cis isomer. This leads to the activation of a photoreceptor channel, as a result, a change occurs in membrane potential and cellular calcium ion concentration. The Photoelectric signal transduction alters the flagellar strokes and results in cell movement.
- It detects the light intensity and direction.
- It helps in the movement of Chlamydomonas towards the light source.
Types of Eyespot
There are present 5 types of eyespots such as Type A, Type B, Type C, Type D, and Type E.
- Type A
It is a part of chloroplast but not associated with flagella. Example: Chlorophyceae and cryptophyceae.
- Type B
It is a part of chloroplast but not associated with swollen flagella. Example: chrysophyceae ,xanthophyceae, phaeophyceae.
- Type C
These are independent clusters of osmophilic granules. Example: euglenophyceae.
- Type D
These are Osmophilic granules of eyespots and conatins membraneous structures Example: Dinophyceae.
- Type E
These are the largest eyespots composed of lens,retinoid and pigmented cups. Example: warnowiaceae family of dinophyceae.